1. Field of the Invention
This invention pertains to a hydraulic system that supplies fluid to a heat source and a circuit that requires a continual source of pressurized hydraulic fluid. More particularly, the invention is concerned with directing fluid in a cooling circuit to a heat exchanger or directly to a heating load on the basis of fluid temperature.
2. Description of the Prior Art
U.S. Pat. No. 4,418,777 describes a powertrain having a high speed a.c. motor, automatic transmission and final drive arranged on a common axis. A hydraulic supply, driven by a small motor, operates only occasionally to accommodate system leakage. A second electrically driven oil pump circulates oil to an oil cooler and supplies the lubrication system of the motor and transmission. A cooling circuit maintains the temperature of the electric motor within an acceptable range. Both. pumps draw oil from a common sump that supplies fluid to the lubrication and cooling systems.
The stator of the motor is cooled by oil flowing from the rotor radially outward on the ends of the stator windings through the action of centrifugal force. The cooling fluid absorbs heat from the motor and returns by gravity to the sump from which the oil is carried to the oil cooling system. The oil cooler includes a small radiator with an electrically driven cooling fan that is activated upon demand to control the oil temperature. Heat from the oil is delivered to atmosphere or can be used to heat the passenger compartment.
Fluid returning to the motor-transmission assembly from the oil cooling system circulates within the transmission to lubricate surfaces requiring lubrication.
The traction motor cooling system described in the '777 patent may reduce traction motor efficiency because of a reduction in the flux path area and will not cool a traction motor when vehicle speed is low. The cooling medium may remain at a low temperature for an extended period of time because all the lubrication and cooling fluid must pass through an external cooler where heat is exchanged between the circulating fluid and ambient atmosphere.